The present invention relates to a multi-band antenna employing a single antenna element adapted to operate in multiple frequency bands.
Recent mobile communication has developed rapidly. Among others, mobile phones have proliferated outstandingly and improvements have been made to reduce their size and weight significantly. According to mobile phone standards, two particular frequency bands are used respectively in different regions: in Japan, a 800 MHz band and a 1.5 GHz band for Personal Digital Cellular (PDC); in Europe, a 900 MHz band and a 1.9 GHz band for Global System for Mobile Communications (GSM); and in North America, a 800 MHz band for Advanced Mobile Phone System (AMPS) and a 1.9 GHz band for Personal Communications System (PCS). Moreover, communication systems such as Global Positioning System (GPS) using 1.5 GHz, Bluetooth using a 2.4 GHz band, and International Mobile Telecommunications (IMT) 2000 using a 2 GHz band are put in practical use for mobile communication and data transmission. If a single antenna is capable of operating in the above-mentioned multiple frequency bands, it would be ideal for the purpose of reducing antenna size and weight.
International Publication No. WO2004/047223A1 discloses an antenna employing a single antenna element adapted to operate in multiple frequency bands. The antenna is configured such that an antenna element has a power-side end which is electrically connected to a power feeding point and intermediate points and a ground-side end which are electrically connected to a ground conductor via switches. By closing one of the switches and opening the remaining switches, it is possible to select the electric length of the antenna element up to the electric connection to the ground conductor from the power feeding point via the closed switch. Further, by selectively controlling the switches, the electric length of the antenna element can be selected, and thus the single antenna element can serve as a multi-band antenna.
Further, instead of the switches, filters may be inserted between the intermediate points and the ground-side end of the antenna element and the ground conductor. The filters are configured so as to allow the passage of only a frequency band where the electric length of the antenna element from the power feeding point to a connection position of the filters corresponds to ½ wavelength of the frequency band. In this case, the filters can serve as the same when the switch is closed only in a passage frequency band, and thus it is possible to allow the single antenna element to serve as a multi-band antenna.
Furthermore, instead of the switches, serial resonant circuits may be inserted between the intermediate points and the ground-side end of the antenna element and the ground conductor. The serial resonant circuits are configured so as to serially resonate in a frequency band where the electric length of the antenna element from the power feeding point to a connection position of the serial resonant circuits corresponds to ½ wavelength of the frequency band. In this case, the serial resonant circuits can serve as the same when the switch is closed only in a passage frequency band, and thus it is possible to allow the single antenna element to serve as a multi-band antenna at the same time.
Furthermore, instead of the switches, a parallel resonant circuit may be inserted between one of the intermediate points and the ground-side end of the antenna element. The parallel resonant circuits are configured so as to resonate in a frequency band where the electric length of the antenna element from the power feeding point to a connection position of the parallel resonant circuits corresponds to ½ wavelength of the frequency band. In this case, the parallel resonant circuits serve as if the switches are closed in a conduction state in a frequency band where electric length of the antenna element with respect to the connection position corresponds to ½ wavelength of the frequency band and prevent the passage of the frequency band where the electric length of the antenna element with respect to the other connection position corresponds to ½ wavelength of the frequency band to serve as if the switches are opened. As a result, it is possible to allow the single antenna element to serve as an antenna for two frequency bands at the same time.
However, with the above configurations, the switches, the filters, the serial resonant circuits, and the parallel resonant circuits should be inserted between the intermediate points and the ground-side end of the antenna element and the ground conductor, and thus spaces for providing them are required, which makes it difficult to reduce the size of the circuit. Further, even though mechanical switches or semiconductor switches are used, the transmission signal is attenuated due to the insertion loss. Further, it is difficult to obtain a sufficient isolation when using the semiconductor switch or filter.